The leakproofness of an object might be necessary or desired to determine for different reasons. For example, the leakproofness might be required to be tested due to environmental concern or quality control. The number of industrial products and utilities required to be tested for leakage has greatly increased during the recent years, which above all depends on increased demands on environmental concern and improved quality. Typical examples of products required to be tested for leakage are parts of refrigeration systems for commercial, domestic and automotive use as well as all types of liquid and gas carrying parts in the automotive industry, such as for example fuel tanks and aluminium wheels.
There are many different methods that can be used for detecting fluid leaks in an object. The two most widely used methods are bubble testing with soap solution and water dunking respectively, wherein the object is submerged in soap solution and water respectively and wherein bubbles then indicate leakage. These two methods are simple, low cost methods, but with restricted sensitivity and high operator dependence. More advanced methods include pressure decay techniques and the below described tracer gas techniques.
All of the above mentioned methods are based on the principle that the object under test or an ambient space of the object is pressurized with a gas, which often is air, and that the gas flows through any leak. When the gas escapes on the low pressure side it is detected in some way. The detection can be done through detecting bubbles visually or by means of some type of instrument detecting pressure, flow or the actual presence of the leaking gas.
In use of methods for leakage testing based on tracer gas techniques, a tracer gas is used for detecting leakage. Such methods generally employ a gas or gas mixture that can be detected after leak passage by means of a detecting instrument. The most commonly used tracer gas today is helium, which typically is detected by a mass spectrometer. Other common tracer gases are refrigerants, sulfur hexafluoride and carbon dioxide.
In some tracer gas methods some type of chamber or enclosure is placed around the complete object or a part of the object under test. The tracer gas is then either added within the object or within the enclosure. Thus, the enclosure is either used to collect any tracer gas leaking from the object or is filled with tracer gas, whereby any leaking tracer gas leaks from the enclosure into the object and any leaking gas is collected inside the tested object. The place where the tracer gas is added, i e within the object or within the enclosure, and thus the direction of any leak flow of tracer gas is decided from case to case and depends on the object to be tested.
One common method today for detecting any tracer gas leaking out into the enclosure or into the object that is relevant to this invention is the so-called accumulation method. For simplicity the principles of this method is only described below for filling the object with tracer gas and detecting any leaking gas in the enclosure, but it is of course also suited for filling the enclosure with tracer gas and detecting any leaking gas within the object.
In the accumulation method any tracer gas that leaks from the object filled with tracer gas is allowed to accumulate in the enclosure during a pre-set time denoted as accumulation time. The concentration of tracer gas in the enclosure increases with time and depends on the leak flow of tracer gas and the volume of the void in the enclosure in which the tracer gas is allowed to accumulate. The volume of the void is denoted as dead volume in the following.
The concentration of tracer gas in the dead volume develops according to the following equation:
      C    tracer    =                              t          acc                *                  C          mixtracer                            V        test              *          F      leak      where:    Ctracer=average tracer gas concentration in the dead volume.    tacc=accumulation time.    Cmixtracer=tracer gas concentration in tracer gas mix (if a mix of tracer gas is used)    Vtest=dead volume in which the leaking tracer gas is accumulated    Fleak=leak flow
As can be seen from this equation, the average tracer gas concentration is directly dependent on the dead volume in the accumulation method.
High sensitivity and high test speed are usually the most important factors to achieve by the used method when determining tracer gas leakage. From the equation above it can be realized that the major parameter limiting the sensitivity as well as the test speed using the accumulation method is the dead volume. Accordingly, in order to achieve as high sensitivity and as high test speed as possible, the dead volume should be as small as possible.
For leakage testing using the accumulation method, the dead volume can be reduced strictly geometrically by building an enclosure that fits more tightly around the test object. However, the cost for geometrical volume reduction increases rapidly with smaller volume and particularly if the geometry of the test object is complex.
Another way of reducing the dead volume for leakage testing using the accumulation method is to reduce the total gas pressure in the volume. This is commonly known as vacuum chamber testing and is widely used in combination with mass spectrometers, which are well suited for vacuum chamber testing since they operate at high vacuum and therefore can be directly applied to the vacuum chamber. The dead volume scales proportionally with the absolute pressure in the chamber. The concentration of tracer gas in the dead volume then develops according to the following equation:
      C    tracer    =                              t          acc                *                  C          mixtracer                *                  P          atm                                      V          test                *                  P          test                      *          F      leak      where:    Ctracer=average tracer gas concentration in the dead volume    tacc=accumulation time    Cmixtracer=tracer gas concentration in tracer gas mix (if a mix of tracer gas is used)    Vtest=dead volume in which the leaking tracer gas is accumulated    Ptest=absolute pressure in the dead volume    Patm=atmospheric pressure (or ambient pressure)    Fleak=leak flow
As can be seen from the equation above, the influence of the dead volume has been reduced by the ratio between the pressure in the dead volume and that of the ambient. The dead volume apparent after reduction of the total gas pressure in the volume is in the following denoted as effective dead volume.
Systems reducing the dead volume by lowering the pressure in the chamber generally employ pressures low enough to ensure high speed mobility of the gas molecules in the dead volume of the chamber. Such pressure is herein denoted as high vacuum. By having high vacuum in the chamber the gas molecules will move at high speed from the leak point to the detector. Another benefit of using high vacuum is that the effective dead volume often is so small that accumulation is not needed.
However, high vacuum systems require advanced pumps and valves that are both delicate and costly. Experience shows also that such systems are very difficult and costly to maintain in normal industrial environments. Furthermore, mass spectrometers which often are used for detection in vacuum chamber testing are very expensive and complicated and therefore costly to maintain. Thus, both capital investments and maintenance costs for vacuum chamber testing are high.
By comparison, systems for testing at atmospheric pressure are cheaper to build and maintain. This is due to the fact that costly vacuum pumps and valves are not needed, but also to that cheaper electronic leak detectors can be used instead of the highly complicated mass spectrometers. However, testing at atmospheric pressure implies a low test speed and a relatively low sensitivity if the dead volume is not reduced. Since the costs for geometrical volume reduction of the dead volume in the enclosure, as mentioned above, is high, testing at atmospheric pressure is generally not suitable for determining leakage at the low leak limits defined in the refrigeration industry and neither for large objects, such as fuel tanks and aluminium wheels.
The most commonly used tracer gas, helium, is a relatively expensive gas and is not a renewable natural resource. Furthermore, it is commonly known that if helium is spilled in or around testing equipment, helium tends to dwell on surfaces and generates thereby increased background signals or false leak signals. Thus, spilling of helium can lead to minutes or even hours of waiting for the gas to dissipate before the testing equipment can be used again.
Thus, in leakage testing using the accumulation method at low pressure, i e high vacuum, it is possible to achieve relatively high test speed as well as high sensitivity, but then the equipment and maintenance costs are high. Leakage testing using the accumulation method at atmospheric pressure is compared to leakage testing at low pressure associated with a lower cost but also lower test speed as well as lower sensitivity.
Accordingly, there is therefore a need for an improved system and an improved method for achieving high test speed and high sensitivity as well as low equipment and maintenance costs for leakproofness determination when the object to be tested is enclosed in an enclosure and a tracer gas is used.